Add [chunkhash] to the output filename when emitting css assets

[moretti]

Add [chunkhash] to the css output filename, just like for JavaScript assets:

gatsby/packages/gatsby/src/utils/webpack.config.js

Lines 116 to 118 in 7572e0d

	case `build-javascript`:
	return {
	filename: `[name]-[chunkhash].js`,

for predictable long term caching.

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[moretti]

Oops, I actually didn't realize that CSS are included in the HTML page, closing this.